Correct way to measure SG

WaterWheel · January 2016

Can someone tell me the correct way to measure SG?

I equalized my almost fully charged fairly new battery bank (3 months old) for 3 hours (63.5v eq voltage with bank temperature starting at 50 degrees) and then allowed the bank to sit with no load for 2 hours before testing. Cells were lightly bubbling and went up to 62 degrees during equalization. The battery bank (Crown L-16s 395 amp hr) was 56 degrees during testing with a Midnight solar hydrovolt hydrometer which is suppose to automatically correct for temperature.

Readings were between 2.50 and 2.55 across 9 cells at different points in the 48v bank. I didn't test all cells because I suspected the readings were too low.

WaterWheel · January 2016

P.S. 2 hrs after equalization with no load the batteries were at 51.4v on 2 different meters with the midpoint voltage at 25.62v on the Conext battery meter.

Vic · January 2016

Hi foolami,

How is it that you are measuring the battery temperature? m a bit surprised that the battery temperature rose that much with a fairly gently 3-hour EQ.

I believe that that Magnin PT100 CC does not allow compensating EQ voltage (?).

If one took the mid-point of the temperature range during EQ, at --0.120 V/C total compensation for the battery, your average EQ voltage was really about 62.1 due to this compensation, and the battery voltage, after the two hour Rest (Rest is with NO charging and NO discharge) the battery V was 51.4 at a nominal battery temperature of about 56 degrees F, means, that the temperature-compensated voltage reading would be about 1.4 volts lower that 51.4 V, or

about 50 V.

A two hour Rest, is a bit short time period, IMO to rest a battery, but, still this does appear that the battery is not quite fully-charged.

You mentioned "readings" of 2.50 - 2.55, assume that these are SG readings, and really 1.250 1.255.

Do you know the Target SG of these Crown batteries? Assume that it might be in the range of about 1.277, or so ...

What is your Absorb voltage? And the Absorb Time?

Does this battery use some water? About what quantity over the three months?

And, in answer to the original question, here is a good treatise on using a Hydrometer:
http://support.rollsbattery.com/support/solutions/articles/4347-measuring-specific-gravity

The Hydrovolt is a bit difficult to clean, so try to fill the Hydro completely, with Distilled water, slosh the water around, expel it and repeat about three additional times.

Thanks, Vic

WaterWheel · January 2016

Thanks for the response Vic,

I have not been able to find the target SC per Crown.

The batteries had only discharged to 82% SOC the previous evening (according the Conext battery meter which I had calibrated a few days before) before bulk charging brought them up to about 91% SOC and then it had gone into Absorb for 2 hours at about 58.4v. At that point with the SOC on the meter showing 100% I switched the house over the grid feed only and started the equalization which ran between 16 and 24 amps into the batteries to maintain the 63.5v voltage.

Over the last 3 months the batteries have used about 8 ozs water per cell.

They are calling for sunny weather all day on Friday and if so I was going to give it another try and need to make sure I'm doing it correctly.

When calibrating the Conext voltage meter I usually wait 3 hours after isolating the battery bank before pushing the "calibrate button". At this time (after 3 hrs) the battery bank is reading 51.2-51.4v so I'm assuming the Conext battery SOC meter is fairly close.

When using the hydrovolt when moving to the next cell I suck in and push out the electrolyte 3 times before taking the reading in an effort to get accurate SC reading.

Vic · January 2016

Hi foolami,

According to this data sheet, the Target SG is 1.275, assuming that your batteries are 6CRPS25s:
http://www.me-power.com/Files/6CRP525.pdf

Good that you are doing some cycling of the battery.

For a purely off-grid system, that 8 oz of water usage would probably be considered a bit low for L-16 batteries, but for a backup system this is probably fine ... just like to see some consumption.

Do you know if you can set the EQ voltage in your CC to be Temperature Compensated? If not, you should try to manually compensate, based on battery temperature, which might be visible from some part of your system.

Another thing, is noting the current going into the battery when you feel the EQ is finished ... you may not have an easy way of measuring this, although, you did note a range of EQ currents ...

Sounds like you are doing the SG measurement correctly. As long as you are carefully cleaning the Hydrovolt when your measuring session is done, you should be fine.

It is good that you are paying attention to the batteries, this is one thing that many of us miss, until something goes wrong.

Thanks for the info, Vic

WaterWheel · February 2016

I'm back, sorry but it's been a busy week and thank you Vic for your insights.      I'd been looking for the Crown spec sheets without luck; thanks for finding them.

Voltage measured with 3 meters and all match within .1v.       SG measurements are carefully taken with 3 draws per cell.     Hydro-Volt cleaned with distilled water after every testing session.

Went through Bulk and then Absorb and into float (battery temp after Absorb 57 degrees) before hitting Equalize on the PT-100 for 3 hours.      The PT-100 maintained 62.5v at about 14 amps for 3 hrs. at which point SG had basically stopped rising.      Let the batteries rest for 2 hrs at which time battery bank voltage was 52.7v and 72 degrees.        SG was a bit higher than last time ranging from 1.254 to 1.261.       So I connected the XW 6848 inverter to the grid and hit equalize.    (Yep, I equalized twice and wil never do that to my batteries again) The inverter gave me 1 hr (automatic cut off) holding 64.2 volts at about 28 amps.     I then let the totally isolated battery bank rest for 4 hours before taking the following measurements using the Hydro-Volt.        after resting bank was 51.2v at 73 degrees
Cell 1 is closest to the + and Cell 24 is closest to the ground.

1.   1.263        2.   1.265        3.   1.265                                                                                                                                      4.   1.254        5.   1.254        5.   1.258                                                                                                                                      7.   1.256        8.   1.262        9.   1.256                                                                                                                                      10. 1.260        11. 1.261        12. 1.259                                                                                                                                      13. 1.260        14. 1.255        15. 1.254                                                                                                                                      16. 1.254        17. 1.252        18. 1.255                                                                                                                                      19. 1.260        20. 1.259        21. 1.260                                                                                                                                      22. 1.261        23. 1.261        24. 1.261

At this point I suspect that while the Hydro-Volt is consistent it is reading low.       Should I try a different SG meter?       Perhaps return the Hydro-Volt for a replacement?

WaterWheel · February 2016

Well, I tried hard to format the cells to be easy to read. best I could do.

westbranch · February 2016

I found using excel or a similar program best as an attachment or copy and paste... not sure how Vanilla likes them though.... hth

Vic · February 2016

Hi again foolami,

Thanks for the date -- the SG spread in your table is not that great about 1.252 - 1.265.

In re-reading this Thread, believe that your Absorb voltage is about 58.4 V. IMO, this probably should be bumped up to about 59.0-ish volts, and make sure that this is Temperature Compensated.

Are you using a Battery Temperature Sensor (BTS) on all charge sources?

EQ can help fully-charge Flooded batteries, but it this is happening, the Absorb time. and/or voltage should be increased.

Would recommend forcing some additional discharge/charge cycles on this battery bank, as, this will help build Capacity, and to help the battery settle-in, and become more predictable.

Is the PT-100 Temperature compensating its charge voltages? Is the XW inverter temperature compensating its charge voltages?

More Later, Thanks, Vic

WaterWheel · February 2016

Vic, The PT-100, XW6848, and the Conext battery monitor are all temperature compensated. Without a Magnum inverter I can't communicate with the PT-100 to increase the Absorb voltage. I guess I should have gone with the Conext charge controllers. The battery bank goes between 84% and 61% OD nightly (62% after a cloudy day) with the average being around 81% DOD in the morning so the bank is being cycled to an extent every night. Should I purposely drain the batteries lower a few times to make them cycle more? I can just flip the panels off for a day to avoid charging for a day occasionally. As you can see I'm a bit over-paneled.

Yes, I can see where the PT-100 is compensating for temperature differences.

WaterWheel · February 2016

Currently the Absorb time is regulated by the PT-100. It automatically holds the batteries in Absorb the same amount if time they take to reach about 51v in bulk. I can manually set the PT-100 to hold absorb longer, perhaps 4-5 hours??? I usually stay in bulk about 3.5 hrs on a sunny morning this time of year. zipcode 30558

WaterWheel · February 2016

I ordered a second Hydro Volt SG meter. A quick test shows a solid 15 point difference in readings between the meters.

No equalization but at the end of a sunny day with light load on the batteries the old meter shows 1.258 the new meter shows 1.274. Similar results on 5 different cells although I'm still showing a wide spread on different cells in the same battery. I'll do a eq and complete testing in a few days.

SolarMusher · February 2016

Hi Foolami,
Did you see the end current for these Crown CRP525 at the end of the absorb charge at 58.4V temp compensated?
What was this current?
Thanks,
Erik

Anawa · February 2016

Foolami, what lead you to suspect your Hydrovolt was mis-reading the SG? I'm been experiencing low SG readings and have wondered about my Hydrovolt.

Paul
in Georgia

WaterWheel · February 2016

Anawa, I have tested both Hydra Volts on several cells now and am convinced the 1st one (2 months old) is reading a solid 15 points lower than the newer one I received the other day. I always rinse them out after a testing session with distilled water.
1.262 vs 1.277 on the same cell is typical between the 2 units.

WaterWheel · February 2016

Eric, At the end of absorb today at 59 degrees absorb voltage was running at 59.6v at 4.1 amps. I then preformed equalization testing SG every 30 minutes before it leveled off after 2 1/2 hrs and I stopped equalization at 3 hrs. EQ voltage after 20 minutes was 63.2v and ended 3 hrs later at 62.5v (14 amps) with an ending temp of 70.

After 4 hr rest the cells with the new meter were around the 1.277 manufacturers specs although there was still more spread than I'd like to see. The battery hooked to the + line was much higher SG than the other batteries and the electrolyte was beginning to look a tiny bit cloudy. Battery wires are heavy welding cables professionally crimped.

SolarMusher · February 2016

foolami said:

Eric, At the end of absorb today at 59 degrees absorb voltage was running at 59.6v at 4.1 amps. I then preformed equalization testing SG every 30 minutes before it leveled off after 2 1/2 hrs and I stopped equalization at 3 hrs. EQ voltage after 20 minutes was 63.2v and ended 3 hrs later at 62.5v (14 amps) with an ending temp of 70.

After 4 hr rest the cells with the new meter were around the 1.277 manufacturers specs although there was still more spread than I'd like to see. The battery hooked to the + line was much higher SG than the other batteries and the electrolyte was beginning to look a tiny bit cloudy. Battery wires are heavy welding cables professionally crimped

Thank a lot Foolami, great info on the 6CRP525!
Erik

Correct way to measure SG

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